Spyridon Mavrakos

Behaviour of vertical bodies of revolution in waves

Abstract This paper presents the so-called macroelement method by means of which the complete linear hydromechanic analysis of arbitrarily shaped bodies of revolution with vertical axis can be carried out. The development of a special method for this wide class of structures which are common in offshore designs is of great advantage for the engineering work. The method described here is based on the discretization of the flow field around the structure by means of ring-shaped macroelements, the velocity potential in each element being approximated with Fourier series. For the matching of the solution between neighbouring elements Galerkins method is applied. Both the diffraction and the radiation problems are solved.

Wave–current interaction on a vertical truncated cylinder floating in finite-depth waters

A new method is developed for solving the diffraction problem around a fixed truncated circular cylinder which is exposed to the action of coexisting wave and current fields with a small but finite current velocity. Linear potential theory is applied and the solution is obtained by using a perturbation expansion for the diffraction potential with respect to the normalized current speed. Because of the axisymmetric geometrical configuration of the examined body, a semi-analytical formulation is used for the treatment of the inhomogeneous free-surface boundary condition involved in the hydrodynamic problem formulation for derivation of the associated perturbation potential. This method results in a Sturm–Liouville problem, which requires the construction of the appropriate Green’s function. The hydrodynamic forces are obtained after evaluation of the pressure field around the cylinder. The calculated results compare very well with numerical predictions of other investigators and existing experimental data. Finally, the mean second-order drift forces are calculated by superposing on to their zero-current values the corresponding current-dependent first-order corrections, the latter being evaluated using a ‘heuristic’ approach.

Parametric Evaluation of the Performance Characteristics of Tight Moored Wave Energy Converters

The paper aims at presenting a numerical model to predict performance characteristics of tight moored vertical axisymmetric wave energy converters that are allowed to move in heave, pitch and sway modes of motion. The hydrodynamic characteristics (exciting wave forces, hydrodynamic parameters) of the floats are evaluated using a linearized diffraction–radiation method of analysis that is suited for the type of bodies under consideration. According to this method matched axisymmetric eigenfunction expansions of the velocity potentials in properly defined fluid regions around the body are introduced to solve the respective diffraction and radiation problems and to calculate the floats’ hydrodynamic characteristics in the frequency domain. Based on these characteristics, the retardation forcing terms are calculated, which account for the memory effects of the motion. In this procedure, the coupling terms between the different modes of motion are properly formulated and taken into account. The floating WEC is connected to an underwater piston that feeds a hydraulic system with pressurized fluid. Numerical results showing parametrically the performance characteristics in terms of the expected power production for several types of floats that are exposed to the wave climate conditions commonly encountered in the Mediterranean area are presented and discussed.Copyright

WAVE RUN-UP AND SECOND-ORDER WAVE FORCES ON A TRUNCATED CIRCULAR CYLINDER DUE TO MONOCHROMATIC WAVES

The second-order diffraction potential around a truncated cylinder is considered. The solution method is based on a semi-analytical formulation for the double frequency diffraction potential. The later is properly decomposed into three components in order to satisfy all boundary conditions involved in the problem. The solution process results in a Sturm-Liouville problem for the ring-shaped outer fluid region, which is defined by the geometry of the structure. The matching of the potentials along the boundaries of neighborhood fluid regions is established with the aid of the ‘free’ wave component. The calculation of integral of the pressure distribution on the free surface is carried out using an appropriate Gauss-Legendre numerical technique. The efficiency of the method described in the present is validated through comparative numerical results.Copyright

Experimental investigation on mooring loads and motions of a TLP floating wind turbine

This paper investigates numerically and experimentally the behaviour of a Tension Leg Platform (TLP) floating wind turbine, under regular waves. The physical model tests have been performed at the Danish Hydraulic Institute (DHI) offshore wave basin. The general aim of this work is to form an accurate set of data, basically consisting in incident waves, 3D wave oscillation and mooring loads that may be used as a possible term of reference for numerical models. The paper provides a thorough description of the geometrical and dynamic characteristics of the floating body, and the results in terms of the incident waves interpreted on the basis of an array of resistive wave gauges, the displacements as observed by a tracking camera system (surge, sway, heave and roll, pitch, yaw), and the loads measured by a load cell located at the fairlead of the anchoring lines, simulating in the physical model scale the tendons characteristics in full scale. Experimental results are given in graphical form for the case of regular wave conditions. Corresponding numerical results that have been obtained using home developed software for the hydrodynamic- and the static- and dynamic-analysis of the mooring lines (HAMVAB [1] and STATMOOR [2], respectively) compare well with the experimental data. STATMOOR [2] is capable to handle the static analysis of extensible mooring lines made of several segments each of them with different geometrical properties and with attached submerged buoys along them.

Berechnungsverfahren für Rotationskörper mit Vertikaler Achse

Wie bereits in der Einleitung dargelegt, kann fur die breite und gerade in meerestechnischen Konstruktionen oft auftretende Familie der Rotationskorper mit vertikaler Achse ein spezielles Verfahren aufgestellt werden.

Berechnungsmethoden für Körper Beliebiger Geometrischer Form

In den letzten Jahren wurde die Entwicklung von Methoden zur potentialtheoretischen Analyse von Korpern beliebiger geometrischer Form stark vorangetrieben, wobei der Einsatz leistungsfahiger Rechenanlagen die Anwendung der entsprechenden numerischen Verfahren ermoglichte (/7/,/16/).

Gültigkeitsbereiche der Diffraktionstheorie

Im Zusammenhang mit der Frage der Anwendbarkeit der potentialtheoretischen Verfahren ist es zweckmasig, sich einen Uberblick uber die Bereiche der relativen Signifikanz der fur die Wellenkraft masgebenden Effekte zu verschaffen.

Belastungen und Bewegungen

Die auf einen Korper durch eine Elementarwelle induzierten Krafte lassen sich uber den instationaren Druck